ALBERT

Beschreibung: Historical data from the region between the Greenwich meridian and the African continental shelf are used to compute the offshore geostrophic transport of the Benguela Current. At 32°S, the Benguela Current is located near the African coast, transporting about 21 Sv (1 Sv = 106 m3 s−1) of surface water toward the north relative to a potential density surface lying between the upper branch of Circumpolar Deep Water and the North Atlantic Deep Watar. Two warm core eddies of probable Agulhas Current origin an observed west of the Benguela Current at 32°S. Near 30°S, the Benguela Current turns toward the northwest and begins to separate from the eastern boundary. It carries about 18 Sv of surface water across 28°S. The current then turns mainly toward the west to flow over a relatively deep segment of the Walvis Ridge south of the Valdivia Bank. A surface current with northward surface of about 10 cm s−1 flows along the western side of the Valdivia Bank, while another northward surface current flows at about 20 cm s−1 some 300 km west of the bank. About 3 Sv of surface now do not leave the Cape Basin south of the Vaidivia Bank, but instead drift northward as a wide. sluggish flow out of the northern end of the Cape Basin. Because of the more southerly seaward extensions of most of the Benguela Current, there are no deep-reaching interactions observed between this current and the cyclonic gyre in the Angola Basin east of the Greenwich meridian. Beneath the surface layer, about 4–5 Sv of Antarctic Intermediate Water are carried northward across 32° and 28°S by the Benguela Current, essentially all of which turns westward to cross the Greenwich meridian south of 24°S.

Beschreibung: The existence of energetic anticyclonic mid-depth vortices of Mediterranean Water (meddies) questions the validity of a conventional advective–diffusive balance in the eastern Atlantic subtropical gyre. A mesoscale experiment in the Azores–Madeira region reveals a link of these meddies to large-scale subsurface meanders. For the first time it is shown that meddies may have strong surface vorticity, indicative of a generation process involving the Azores Current—a deep reaching near-surface jet.

Beschreibung: Quasi-homogeneous layers in vertical profiles of temperature and salinity in the eastern North Atlantic near Madeira indicate the existence of a subtropical Mode Water in the Eastern Basin. Temperature sections show a maximum horizontal extent of at least 500 km. The frequency distribution analysis of homogeneous layers in a historical XBT dataset shows a Mode Water formation region near and to the north of Madeira. This Mode Water is found at increasing depths and displaced to the west and southwest during the course of the year after its formation by wintertime convection. It disappears almost completely, due to mixing, before the next winter. Volume estimates suggest that this Madeira Mode Water in the eastern Atlantic accounts for 15–20% of the total Central Water formation in the corresponding density range as obtained from tracer studies in the North Atlantic gyre.

Beschreibung: The inclination of oceanographic mooring lines due to current drag causes errors in time series observations of currents and temperatures. The prediction of this effect requires knowledge of the drag coefficients for the mooring components. Drag coefficients, known for simple geometric shapes such as spheres or cylinders, are commonly used for mooring response computations. Selected mooring components (buoyancy elements and instruments) were tested in a tow tank to determine their actual drag coefficients. Over the Reynolds Number range, typical of oceanic conditions, deviations of the drag coefficient up to 50% are found when compared with the appropriate simple geometric shape coefficients. A set of model moorings and model current profiles is used to determine the resulting changes in component depth level and displacement. The changes in horizontal displacement of the upper part of the mooring are on the order of 10% in extreme cases and 1% under typical conditions. Their effects on current measurements will usually be negligible. However, the related vertical displacements are on the order 100 to 10 m. Such vertical displacements may carry instruments to depth levels where currents and particularly thermocline temperatures are sufficiently different from the intended level to cause errors in the time series observations.

Beschreibung: Long-term temperature and current-meter records from moorings in the northern Canary Basin display strong current events with time scales between one and three months and large vertical scales of several thousand meters. The data are compared to hydrographic surveys in the area that show a meandering subtropical front. The strong current events are found to be related to the passage of the front through the mooring positions. An analysis of composite time series, for selected depths, indicates cases of westward and of eastward propagation of frontal meanders. The frontal pattern is also found in geopotential anomalies inferred from historical XBT data sets, suggesting that the front is a persistent feature of the density field. In two cases strong current events appear to be related to a Mediterranean Water lens.

Beschreibung: Giant petrels are the dominant scavenging seabird species in the Sub-Antarctic and maritime Antarctic ecosystems. They consume large amounts of penguin carrion, but also include significant numbers of seals, Antarctic Krill, Euphausia superba, and small petrels in their diets. Using results of detailed dietary studies at Bird Island, South Georgia, and qualitative data for other localities, notable variation in the quantitative impact of Giant petrels on their prey is revealed. Energy budgets of chicks are calculated from data on meal size and feeding frequency at all stages of growth. Differences in energy requirements between the Northern Giant Petrel, Macronectes halli, and the Southern Giant Petrel, M. giganteus, and between male and female chicks, are demonstrated. Published and unpublished data on numbers of giant petrels at known breeding sites are reviewed, and the present world breeding population is concluded to be ca. 8,600 pairs of M. halli and ca. 38,000 pairs of M. giganteus. Using survival and demographic data for South Georgia, total world non-breeding populations of ca. 26,000 and ca. 113,000 birds are calculated for M. halli and M. giganteus, respectively. The energy consumption of these populations in the breeding season is assessed and, taking into account geographical variation in diet, their impact on local ecosystems is evaluated. The approximations and assumptions of these estimates are discussed.

Beschreibung: The methods and results of the study of the tropho-dynamic relationships between 2 Diomedea albatrosses and the marine environment at South Georgia are described. They illustrate the technical and theoretical developments necessary to obtain certain empirical data essential for accurate assessments of the role of seabirds in marine ecosystems. Differences in breeding success during eight yr (consistent in D. chrysostoma, more variable in D. melanophris) are linked with important differences in breeding frequencies which affect the size and activities of populations at the breeding sites. Extensive dietary studies, based on sampling adults about to feed chicks, showed major inter-specific differences, resulting in chicks receiving meals of similar size and frequency but of different energy content. The frequency of chick feeding was determined initially by daily and 3-h weighing. Recently automatic equipment has recorded weights every 10 min, giving the frequency and size of meals and resulting digestive performances of the chicks. Experiments involving exchanging chicks between the two species were combined with new methods for analyzing growth curves. They showed that, while there was a species-specific genetic component to growth, the overall rate could be significantly modified by the nature of the diet. The slower growth rate of D. chrysostoma chicks, and the species’ diet, are probably important factors affecting breeding frequency. Adult feeding performance is being studied by devices recording simple activity budgets at sea. Preliminary results are described and projected work linking this with the automatic weighing equipment and with assessment of foraging energy costs is outlined.

Beschreibung: The energy requirements and prey consumption of the rapidly expanding South Georgia population of Antarctic Fur Seals, Arctocephalus gazella, are estimated, using information on diet, population structure, activity patterns, and energy budgets and variations in these with sex and season. All quantitative information on diet is reviewed. Breeding female Fur Seals mainly take Antarctic Krill, Euphausia superba. This predominates also in the summer diet of males but fish (mainly Champsocephalus gunnari) and squid are also eaten. Fish is more important to males in winter. Life tables are used to determine the proportion of each age-class in the population, and especially to estimate the size of that portion of the immature stock which does not appear ashore. Energy costs of adult males are assessed from attendance data and age-specific weights. Female energy budgets are calculated from extensive data on the duration of periods ashore and foraging trips to sea together with information on activity budgets at sea. Pup energy consumption is calculated from body weight and growth data. A summer food consumption of 1.1 × 106 t is divided between juvenile males (39%), adult females (31%), adult males (22%) and juvenile females (8%). In winter, when females are absent, juvenile (67%) and adult males (33%) consume 0.4 × 106 t. Krill forms 69% of the annual consumption, fish 19% and squid 12%. The main peaks of demand are in October (all adults feeding at sea), January-February (rapidly growing pups being reared) and April—May (most of population, including weaners, feeding at sea). From data on diet and foraging ranges and depths, the main natural competitor of the Antarctic Fur Seal is the Macaroni Penguin, Eudyptes chrysolophus; significant competition with a developed Krill fishery would be expected. Continuing commercial exploitation of fish, especially C. gunnari, may influence male Fur Seal populations, particularly in winter.

Beschreibung: This paper reviews the current state of knowledge of the food habits of the poorly known small cetaceans of the Southern Ocean and presents new information obtained through a study of stranded specimens in Tierra del Fuego. In the Antarctic and Sub-Antarctic, food data are available for only 10 of the 22 species present. Adding to this all the food records known from the South Temperate Zone south of 30° S, there is still no information at all for 5 species (Mesoplodon layardii, M. hectori, M. bowdoini, Cephalorhynchus eutropia and Phocoena dioptrica); that of 8 species is based on fewer than three samples, and more than 10 samples are available for only 2 species (Orcinus orca and Cephalorhynchus commersonii. Since these cetaceans are among the larger animals of the Southern Ocean, their role in the food web of the ecosystem cannot be determined until more information is available.

Beschreibung: The sensitivity of the global climate system to interannual variability of he Eurasian snow cover has been investigated with numerical models. It was found that heavier than normal Eurasian snow cover in spring leads to a “poor” monsoon over Southeast Asia thereby verifying an idea over 100 years old. The poor monsoon was characterized by reduced rainfall over India and Burma, reduced wind stress over the Indian Ocean, lower than normal temperatures on the Asian land mass and in the overlying atmospheric column, reduced tropical jet, increased soil moisture, and other features associated with poor monsoons. Lighter than normal snow cover led to a “good” monsoon with atmospheric anomalies like those described above but of opposite sign. Remote responses from the snow field perturbation include readjustment of the Northern Hemispheric mass field in midlatitude, an equatorially symmetric response of the tropical geopotential height and temperature field and weak, but significant, perturbations in the surface wind stress and heat flux in the tropical Pacific. The physics responsible for the regional response involves all elements of both the surface heat budget and heat budget of the full atmospheric column. In essence, the snow, soil and atmospheric moisture all act to keep the land and overlying atmospheric column colder than normal during a heavy snow simulation thus reducing the land–ocean temperature contrast needed to initiate the monsoon. The remote responses are driven by heating anomalies associated with both large scale air-sea interactions and precipitation events. The model winds from the heavy snow experiment were used to drive an ocean model. The SST field in that model developed a weak El Niño in the equatorial Pacific. A coupled ocean-atmosphere model simulation perturbed only by anomalous Eurasian snow cover was also run and it developed a much stranger El Niño in the Pacific. The coupled system clearly amplified the wind stress anomaly associated with the poor monsoon. These results show the important role of an evolving (not specified) sea surface temperature in numerical experiments and the real climate system. Our general results also demonstrate the importance of land processes in global climate dynamics and their possible role as one of the factors that could trigger ENSO events.